1. Field of the Invention
The invention relates generally to the field of illuminators for small displays. More particularly, the invention relates to a compact light source using a film stack to illuminate a small display.
2. Description of the Related Art
Micro displays of various kinds provide compact high resolution displays for a variety of applications. Such displays are used, for example in helmet, windshield, and visor projection systems as well as in small portable headsets and handsets for private display viewing and for virtual reality systems. Some micro display technologies, such as polysilicon (P-Si) and silicon-on-insulator (SOI), provide a transmissive surface which requires a backlight for illumination. Other types, such as Nematic, Static Random Access Memory (SRAM), Digital Micromirror (DMD), Ferro-electric (FLC) and Liquid Crystal on Silicon (LCoS) are reflective and require a front light. In any such micro display system, there are significant challenges in making a system that is both compact and low powered. Both challenges are eased by reducing light loss. Reducing light loss in the illumination system, whether it is a backlight or front light, allows for a smaller, cooler, lower power illumination system.
One example of an illuminator for a micro display is shown in U.S. Pat. No. 5,684,354 to Gleckman. That patent shows an illuminator with either a bulb 12 or red, green and blue LEDs 12xe2x80x2 disposed within a reflective cavity to illuminate the display. An opaque baffle 56 or a wall of the cavity keeps the light source out of the field of view of the display and the optical system to ensure that the light source is not imaged to the viewer. The illumination light comes out of the reflective cavity through an aperture 18 that is covered by a diffuser 20, a film 22 and a corrective color filter 24, if necessary. This illuminator provides an even, well diffused, color corrected source of light for the display. However, the cavity must be large in order to keep the light source out of the view of the display and imaging optics. In addition, the illuminator is inefficient due to the indirect path from the light source to the display.
A compact and efficient illumination system is provided that is particularly suitable for illuminating a micro display and places the light source within the view of the display. In one embodiment, the illumination system comprises a light source, a package for the light source having a reflective surface on at least one side of the light source, and a diffuser opposite the reflective surface in direct view of the light source. The invention further comprises a film stack to receive light through the diffuser, the film stack transmitting light within a maximum exit angle and reflecting substantially all other light to the diffuser.